1. Field of the Invention
The present invention relates generally to light-emitting devices and, more particularly, to light emitting devices with reduced light piping for high efficiency AlGaInP-based light emitting diodes (LED""s).
2. Discussion of the Prior Art
High efficiency AlGaInP-based amber and red-orange light emitting diodes (LED) are important in applications such as the large area display, traffic signal lighting and automotive lighting. The luminescence performance of the visible LED is determined by the internal quantum efficiency and the light extraction efficiency. High quantum efficiency has been demonstrated in double heterostructure (DH) and quantum well (QW) LED. On the other hand, the light extraction efficiency of the LED is limited by the substrate absorption, the internal reflection at the chip surface and the current crowding under the contact electrode. For example, early AlGaInP LED using a circular ohmic contact has a low efficiency of 0.4 lm/W. The light emission in this device is confined to near the edge of the contact since the majority of the light is generated under and blocked by the contact metal. The prior art approach includes the use of a window layer, a current-blocking layer, a transparent conductive oxide layer and a textured surface to improve the light-extraction efficiency of the LED.
FIG. 1 is a prior art DH-LED on a transparent substrate using a window layer disclosed by F. Kish et al in U.S. Pat. No. 5,793,062. The LED contains a thick GaP window layer allowing the emitted light to escape from the top with reduced total internal reflection loss. The AlGaInP DH 12 are grown on GaAs substrate using the metalorganic vapor phase epitaxy (MOVPE) method comprising an AlGaInP lower confining layer 120, an AlGaInP active layer 122, and an AlGaInP upper confining layer 124. However, the prolonged growth cycle of the 50-um thick GaP window layer 14 at a high temperature causes deterioration of the impurity doping profile and adds cost to the LED wafer. This requires the use of a second crystal growth method such as vapor phase epitaxy (VPE) to deposit the thick window layer due to the high growth rate of VPE for the growth of thick layers. In the transparent substrate (TS) LED design, the LED layer is first grown on GaAs substrate and then lifted off and bonded to a second non-absorbing substrate such as GaP 11. A p-electrode 26 is deposited on top surface of the wafer and an n-electrode 28 is deposited on the back surface of the wafer. The TS-LED, in conjunction with a thick window layer, has been reported to show the best luminescence efficiency to date. The wafer bonding process, however, requires special attention to avoid the inclusion of foreign particulate and to reduce the build up of thermal stress during the bonding and the subsequent annealing process. The process yield is sensitive to the bonding parameters and adds extra cost to the wafer.
FIG. 2 is a prior art DH-LED on GaAs substrate 10 using a current blocking layer (CB) 22 and a distributed Bragg reflector (DBR) 20 disclosed by H. Sugawara et al in Appl. Phys.Lett. vol 61 (1992) pp. 1775. The Bragg reflector 20 was grown on GaAs substrate 10, followed by AlGaInP DH 12, and an n-type AlGaInP blocking layer 22. After photolithographic definition of the blocking layer 22, a p-type AlGaAs current spreading layer 24 was grown over the top followed by a p-GaAs contact layer 16. The p-electrode 26 and the n-electrode 28 were formed using AuZn/Au and AuGe/Au, respectively. In this design, a current blocking layer 22 is used for current spreading whereas a DBR 20 is used to reduce substrate absorption loss. High efficiency LED has been achieved using this design. However, this method requires a second growth step after the definition of the current blocking layer. The quality of the high Al-content current spreading layer is sensitive to the oxygen contamination during the regrowth.
FIG. 3 is a prior art LED using a transparent conductive oxide (TCO) layer 30 with a contact layer 16 and a DBR 20 disclosed by B-J. Lee et al in U.S. Pat. No. 5,789,768. The LED structure is grown using MOVPE and contains an AlGaInP or AlGaAs DBR 20 deposited on GaAs substrate 10, an AlGaInP DH 12 deposited on the DBR 20, a Zn-doped GaP window layer 14 on the DH 12, a p-type Zn-doped GaAs layer 16 on the window layer 14, then over-deposited with a TCO layer 30 after an opening is defined in the center of the p-GaAs contact layer 16. The DH 12 comprises an Si-doped AlGaInP lower cladding layer 120, an AlGaInP active layer 122 and a Zn-doped AlGaInP upper cladding layer 124. The GaP window layer 14 is 4-10 um thickness. The p-GaAs contact layer 16 is Zn-doped to 5xc3x971018 cmxe2x88x923 with a resistivity of 0.01 ohmcm. The current injection under the electrode 26 is blocked due to the Shottky contact formation between TCO layer 30 and GaP window layer 14. The injected current diffuses away from the electrode 26 and conducts through the p-type contact layer 16. However, the TCO is an n-type semiconductor and it forms a rectifying contact with p-type semiconductors. The resistivity of TCO is 3xc3x9710xe2x88x924 ohmcm that is two orders of magnitude higher than for a good conductor such as silver. This has limited the use of TCO to reduce the current crowding under the p-electrode 26. FIG. 4 is the calculated current spreading for a 250 umxc3x97250 um die and a contact pad diameter of 84 um. It is shown that a thick ITO layer is needed for efficient current spreading due to the limited conductivity of TCO. However, it is impractical to deposit thick ITO films using the conventional vacuum deposition methods. For the conventional case, the ITO film is reactively deposited at around 60 angstrom/min in 10xe2x88x924 torr oxygen partial pressure. For this reason, the prior art LED still requires the use of a current-blocking layer for current spreading even with a TCO layer on the surface. To correct the problem, an object of the present invention is to use a hybrid TCO/conductor layer for current spreading and surface light-extraction for high efficiency LED applications.
The prior LED design contains a DBR 20 at the lower interface to reduce substrate absorption. However, DBR stack 20 is only reflective for normal light incidence as in the case of the vertical cavity surface emitting laser (VCSEL) applications. FIG. 5 shows the reflectance spectrum of a typical DBR stack comprising a stack of 20 pairs of quarter wavelength GaAs/AlInP layers. The DBR stack is nearly 40% reflective at the design wavelength of 570 nm. Due to its limited bandwidth, a separate DBR is needed for LED emitting a different color. Moreover, due to the non-directional light emission of the LED, the reflective power is much less for light entering the DBR at greater angles. FIG. 6 shows the calculated angular variation of the reflectance of the GaAs/AlInP DBR. The reflectance of the DBR drops rapidly at an incident angle greater than about 20 degree, causing optical loss due to the light transmission into the absorbing substrate.
Even with an ideal DBR at the substrate interface, most of the emitted light is piped to the side of the chip due to TIR at the top surface. Light piping causes multiple absorption loss of the emitted light before it exits to outside of the chip. For this reason, QW LED with very thin active layers is preferred to reduce the light absorption loss in the active layer. In order to minimize absorption loss due to the light piping, another object of the present invention is to reduce the optical loss due to the light piping by maximizing the surface light extraction using a hybrid antireflective layer.
The aforementioned deficiencies are addressed, and an advance is made in the art, by employing, in a light emitting diode structure, a hybrid anti-reflection (AR) and high reflection (HR) layers comprising a TCO layer and a conductor layer for surface light extraction and uniform current injection of the LED.
In accordance with an illustrative embodiment of the present invention, an LED includes a hybrid conductive transparent layer on the top surface and a conductive lower reflecting layer. The top transparent layer comprises a thin conductor layer such as Ag and a transparent conductive oxide layer such as SnO2. The high conductivity of Ag enables uniform current injection and forms ohmic contacts between the conductive oxide layer and the semiconductor. The top oxide layer serves to protect the Ag film from environmental degradation and to promote the anti-reflection for light emission from the LED surface.
The lower reflective layer also comprises a conductor film such as Ag and an upper oxide layer such as ITO. The reflector is first deposited on a second substrate and then wafer bonded to the AlGaInP LED. The downward emitting light is reflected away from the light absorbing substrate and escape at the top surface. The optical loss due to light piping and substrate absorption is thus minimized in the high efficiency LED. The ITO layer serves to prevent the Ag layer from degradation and to promote the adhesion of the layers during the wafer bonding process. The LED in the present invention is advantageous in terms of simple processing and cost effective as compared with the conventional structure using a thick window layer and a current-blocking layer for current-spreading.